Liturgical slippers or sandals are shoes worn by bishops and other prelates during the Solemn Pontifical Mass, though their use is now limited to the celebration of pre-1970 forms of the Mass. They are made of silk and ornamentation depends on the status of the wearer. Their color must match the liturgical color of the day, although popes and cardinals are permitted to wear slippers embroidered with gold or silver, whereas Bishops may only have a strip of gold or silver bordering their slippers.

Sources:

"Pontifical vestments," Wikipedia, December 5, 2023. https://en.wikipedia.org/wiki/Pontifical_vestments.

Tribe, Shawn. “Pontifical Sandals: A Brief History and Consideration.” Liturgical Arts Journal, July 12, 2018. https://www.liturgicalartsjournal.com/2018/07/pontifical-sandals-brief-history-and.html.